If the floor is not level the shower pan will not be able to drain properly.
Install shower over floor drain.
This guide explains how to install a shower pan over existing plumbing.
If the subfloor is wood waterproof it before using self leveler and before spreading mortar or adhesive.
If you want to install a shower over a concrete floor a fiberglass shower stall will work just fine.
It is a great remodeling project and can help to add long term value to your home.
Then install the membrane over the mortar bed and over the drain base and cut out a section over the drain hole in order to attach the center part of the drain on which the weep holes are located.
This limits your ability to change the basement layout but if the stub outs are in place using them will save money and labor.
Ideally you ll already have a drain roughed in with the pipe sticking a few inches above the floor.
Fix an uneven floor using a self leveler.
The most efficient and least expensive method of installing a basement shower without the need to break concrete is to install it over existing plumbing stub outs.